1. Field of the Invention
This invention relates generally to cordless telephones. More particularly, it relates to a technique for avoiding channel interference with an improved channel selection for each of separate digital cordless telephones.
2. Background of Related Art
Cordless telephones are extremely useful devices. The remote handset of a cordless telephone provides a user with the freedom to roam about a house or small office while conversing in a telephone call.
A typical cordless telephone utilizes one communication channel for voice transmissions from the remote handset to the corresponding base unit, and another communication channel for voice transmissions from the base unit to the remote handset. In the case of a narrowband digital cordless telephone, time division multiplexing and/or frequency multiplexing techniques are typically used, providing additional channels, e.g., a control data channel in each direction for passing control data relating to, e.g., tuning of the selected channel, turning the remote handset of the cordless telephone ON and OFF, etc.
Most cordless telephones are capable of operating over any one of a plurality of RF channels in either direction, allowing a xe2x80x9cbest availablexe2x80x9d type option for selection of an RF channel hopefully having the least amount of interference. In early cordless telephones, few RF channels were made available, e.g., only two possibilities. However, in more recent cordless telephones, e.g., digital cordless telephones, the cordless telephone is capable of communication over any one (or more) or a large plurality of available RF channels, e.g., ten (10), twenty (20), sixty-four (64), or more RF channels.
FIG. 3 shows a plurality of conventional digital cordless telephones in operation within an overlapping area of coverage.
In particular, a first digital cordless telephone 400a, 402a, a second digital cordless telephone 400b, 402b, and even a third digital cordless telephone 400c, 402c may be in operation within a limited area, e.g., within a home or small business. In such a case, the range of transmission of each of the respective remote handsets 402a-402c is such that more than just the intended base unit will receive the transmission. The base units of conventional digital cordless telephones will ignore transmissions from remote handsets which do not include a matching security code with that assigned to the respective base unit.
Even with the larger number of available RF channels, improvements in the range of cordless telephone remote handsets have nevertheless increased the probability that two or more cordless telephones will at least attempt at times to operate using the same selected RF channel, causing channel interference on the common selected channel for one or both of the conflicting cordless telephones.
This conflict is exacerbated somewhat by the fact that a consumer will oftentimes buy two or more of the same cordless telephone for installation in a number of locations throughout a home or small office. It is likely that at least two of these installed cordless telephones will be within range of one another. However, often unbeknownst to the consumer, if the cordless telephones are identical, they will most likely use the same channel allocation algorithm for selection of an appropriate RF channel.
For instance, when more than one digital cordless telephone is implemented with a number of RF channels, at least one which overlaps with a same RF channel of another digital cordless telephone, it is likely that the operations of the two cordless telephones will conflict at least with respect to the one overlapping RF channel, possibly preventing remote communications for either or both digital cordless telephone(s).
The RF channel used for communications is controlled by a processor such as a microprocessor, microcontroller, or digital signal processor (DSP).
Moreover, use of the same conventional RF channel selection algorithm without consideration of outside factors is likely to cause assignment of the same RF channel for use by both cordless telephones based on similar criteria, causing deteriorated clarity and even interference in that RF channel. In general, this type of conflicting situation is likely to cause poor communications for both cordless telephones.
FIG. 4 is a detailed block diagram of the relevant portions of a base unit of a conventional digital cordless telephone.
In particular, FIG. 4 shows a conventional base unit 400 which ignores entirely transmissions from remote handsets not including a particular security code assigned to that particular base unit. Channel selection in such conventional base units are performed either randomly, at the direction of the user, and/or based on channel conditions irrespective of the content of transmissions from non-matching, unauthorized remote handsets.
The base unit 400 includes an RF front end capable of transmitting and receiving on any of, e.g., ten (10) sets of RF channels. The available RF channels may be, for example, time division multiplexed channels transmitted at a common frequency, frequency division multiplexed, or a combination of both time division and frequency multiplexing. The particular transmission and receiving frequencies are selected by a channel selector module 460 based on predetermined criteria, e.g., based on current interference conditions in the channels. In many digital cordless telephones, the current channels are selected at random, and changed as interference conditions warrant.
A received security code authorization module 470 retains the unique security code assigned to the particular base unit 400 and its matching remote handset 402 (not shown in FIG. 4). Transmissions received by the base unit 400 are authorized for processing by the received security code authorization module 470 only if they include the appropriate security code matching that is assigned to the particular base unit. If a received transmission does not include the appropriate security code, the received transmission is discarded.
A controller 450 controls the general operations of the base unit 400. The controller is typically any suitable processor, e.g., a microprocessor, a microcontroller, or a digital signal processor (DSP). A telephone line interface (TLI) 440 provides the appropriate AC and DC impedance matching to the telephone in both an on-hook and off-hook condition. The telephone line interface 440 also provides a ring detect signal to the controller 450 upon detection of an incoming ring signal preceding an incoming telephone call.
A codec 430 provides the appropriate analog-to-digital and digital-to-analog conversion between the RF front end 480 and the telephone line interface 440.
RF communication channels in a digital cordless telephone are typically selected by brute force, i.e., by scanning all available RF channels and selecting for use those RF channels with the least amount of interference. Channel scanning is typically performed when the base and handset cease communications. The handset is then in the receive only mode, commonly referred to as the standby mode. Thus, conventional digital cordless telephones select RF channels for communications between the assigned remote handset and its matching base unit based on current conditions of the particular RF channel irrespective of the cause of the interference (e.g., because of another digital cordless telephone within range of the relevant base unit utilizing the same RF channel). As a result, the probability of collisions between separate digital cordless telephones using the same RF channel is significant, particularly as the range of typical digital cordless telephones continues to increase in response to consumer demand.
Thus, there is a need for an improved technique and apparatus for digital cordless telephones to select RF communication channels in such a way as to avoid channel interference caused by range overlap with another digital cordless telephone.
In accordance with the principles of the present invention, an RF channel selector module for a digital cordless telephone comprises a stored list containing an identity of at least one other cordless telephone within a transmission range of the digital cordless telephone. An overlapping channel selector module is adapted to arrange a plurality of best available RF channels based on the identity of at least one other cordless telephone.
A method of selecting an RF channel for a digital cordless telephone based on a presence of other cordless telephones within transmission range of the digital cordless telephone in accordance with another aspect of the present invention comprises determining a list of best available RF channels. The list of best available RF channels is randomly arranged based on unique information regarding the digital cordless telephone.